Exclusive content is an idea by the major cmpanies in order to have more control over the flow of sales to multiplatform titles. They really could not hold back a title from being released on other consoles (unless the company is first/second party). The timed exclusives thing was on the way there, but eventually that agreement ends and the game makes it's way across to other platforms.
With exclusive content, a company such as Sony or Microsoft pays the developer for the development costs of the exclusive content, which in a lot of cases, that exclusive content remains permanent property of the paying company. No timed expiry date or anything.
Personally, I don't see the big deal with exclusive content in most cases. It generally falls into one of two categories:
1. The exclusive content has no effect on factors such as the storyline. It just can't. At most it can append to a story, but it could never interleave with core storyline. Things such as exclusive items tend to damage gameplay experience rather than add to it, since most times the item is generally higher power then regular items, making the game less challenging.
2. Exclusive content that does in fact affect the main storyline usually means that the version without the exclusive content could be seen as incomplete, and damage the publisher's reputation of a robust system.
